The present invention relates to the production of electrical energy by a stand-alone electrical generator, in particular a stand-alone electrical generator of the type used in aircraft.
The invention relates more particularly to a method of controlling a stand-alone electrical generator of the type used in aircraft.
An electrical generator control method is described in an article by Yifan Tang et al. published in IEEE Transactions On Power Electronics, Vol. 10, No. 4, July 1995.
That prior art control method is applied to an asynchronous machine whose stator is connected to the mains electrical power supply and whose wound rotor is driven mechanically. The method uses a constant DC voltage source for electrical excitation of the rotor winding. The DC voltage source feeds the electrical energy into the rotor via an inverter controlled by a pulse width modulation control circuit.
The direct current source of the prior art control device also includes an AC/DC converter or rectifier which is controlled by a pulse width modulation control circuit and which energizes a capacitor connected to the input of the inverter.
The control circuits of the inverter and the rectifier are controlled by control loops designed to control the operation of the generator so that certain operating criteria are complied with. To be more precise, the control system seeks to achieve stable control by controlling the active and reactive power of the generator. The control loops therefore receive set points in the form of an active power reference value and a reactive power reference value.
However, the above control method is designed to be applied to the generators connected to very high power mains electrical power supply networks in which the frequency and the voltage are imposed by the network itself, for example a public mains electrical power supply network.
However, in some applications of such electrical generators the network supplied is relatively small, as is the case in an aircraft, for example, and the voltage and the frequency are not imposed from the outside, but depend on particular operating conditions of the generator, for example the applied load or the rotation speed at which it is driven. In such applications the rotation speed of the device driving the generator can vary considerably. Thus in an aircraft, for example, the speed depends on the rotation speed of the propulsion engines, and can therefore vary by a factor of greater than 2 (from 500 to 1200 radians per second, for example)
The invention aims to provide a method of controlling a stand-alone electrical generator producing electrical energy for a relatively small electrical power supply network at a particular nominal voltage and at a frequency which is constant or variable, depending on the application. The benefit of the invention is that it reduces the range of variation of the frequency relative to the range of variation of the rotation speed.
The invention therefore provides a method of controlling a stand-alone electrical generator including an asynchronous rotating machine whose wound rotor is driven mechanically, in particular to power small electrical power supply networks, at a particular voltage and with a particular frequency, such as those used on aircraft, the stator of said machine being connected to the network and its rotor being excited by an inverter controlled by a pulse width modulation control circuit, the inverter itself being excited by a constant DC voltage source, which method is characterized in that it consists of:
generating a signal representing the stator frequency (or angular frequency) set point,
generating a signal representing the rotor frequency (or angular frequency) set point as a function of the stator frequency set point and the rotation speed of said machine,
generating a signal representing the stator voltage rms set point value,
generating an error signal as a function of the difference between said stator voltage rms set point value and the rms value of the actual stator voltage, and
imposing on said control circuit a rotor voltage set point which is a function of said error signal and said rotor frequency set point.
Because of the above features, the generator can maintain the voltage applied to the network and its frequency at their respective set point values.
These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.